Stopped-Flow and DFT Studies of Proton Transfer and Isomerization of 5-Amino-3-imino-1,2,6,7-tetracyano-3H-pyrrolizine and Its Related Base 2-(5-Amino-3,4-dicyano-2H-pyrrol-2-ylidene)-1,1,2-tricyanoethanide in Water: A Completely Sorted out Square Sc

摘要

A complete kinetic study of pH-dependent isomerization of 5-amino-3-imino-1,2,6,7-tetracyano-3H-pyrrolizine (HL) and 2-(5-amino-3,4-dicyano-2H-pyrrol-2-ylidene)-1,1,2-tricyanoethanide (L'~-) in the pH range from -0.5 to +13 in aqueous solution, which spans over 7 orders of magnitude for the pseudo-first-order rates (log k from -4.8 to +2.5), has revealed for the first time the existence of the corresponding conjugate species L~- and HL'. The study has allowed the determination of the acid dissociation constants of HL (pK_a) and HL' (pK'_a), as well as all of the individual forward and reverse isomerization rates of the acidic (HL/HL') and basic (L~-/L'~-) forms. Although L'~- is the sole species present at pH > ca. 7, species HL and HL' establish an equilibrium favoring the former compound at low pH, with the distribution being ca. 79:21 at pH less than ca. 2. A slow decomposition of HL occurs at low pH via the initial formation of an unobserved and unstable protonated form H_2L~+. Geometry optimizations of all species at the B3LYP/LANL2DZ level, including water solvation energy, afford results in qualitative agreement with the experiment and indicate the structural nature of the new species L~- and HL'.